Delegation for fmt traits (#321)

[tyranron]

Resolves #321

Synopsis

See #321 (comment):

You’re discarding formatting flags provided by the user in format string, e.g.:

#[derive(derive_more::Display)]
#[display(fmt = "{:?}", _0)]
struct Num(usize);

impl std::fmt::Debug for Num {
    fn fmt(&self, fmtr: &mut std::fmt::Formatter) -> std::fmt::Result {
        self.0.fmt(fmtr)
    }
}

fn main() {
    let num = Num(7);
    println!("{num:03?}");  // prints ‘007’ as expected
    println!("{num:03}");   // prints ‘7’ instead
}

Solution

See #321 (comment):

Theoretically, we can support this with the current syntax, because we can detect so-called trivial cases and transform them into delegation (we do parse formatting string literal anyway).

#[derive(derive_more::Display)]
#[display("{_0:?}")] // <--- it's clear to be a trivial delegation case
struct Num(usize);

would expand to

impl std::fmt::Display for Num {
    fn fmt(&self, fmtr: &mut std::fmt::Formatter) -> std::fmt::Result {
        let _0 = &self.0;
        std::fmt::Debug::fmt(_0, fmtr)
    }
}

rather than

impl std::fmt::Display for Num {
    fn fmt(&self, fmtr: &mut std::fmt::Formatter) -> std::fmt::Result {
        let _0 = &self.0;
        write!(fmtr, "{_0:?}")
    }
}

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[JelteF]

When is this "default" case applicable?

[tyranron]

@JelteF when format_spec is not specified, i.e. {} or {ident}. See grammar for details.

[JelteF]

were all these newlines intentional? (feel free to keep if they were)

[tyranron]

@JelteF yes.... 4 newlines before ## headers and 2 newlines before ### headers.

[JelteF]

were all these newlines intentional? (feel free to keep if they were)

[JelteF]

LGTM. Thanks for the extensive test suite!